We continue the study of the development of thymic lymphocytes from bone marrow stem cells. Bone marrow preparations, separated by velocity sedimentation, iso-pycnic sedimentation and counter-current distribution is also being used to repopulate the thymus irradiated mice and the kinetics of the appearance of thymic antigens are being studied both in vivo and cell culture. The development of mature, functional thymic lymphocytes will be correlated with these antigenic changes and the influence of a thymic humoral factor on these changes will also be evaluated.